Question

A magnetised needle in a uniform magnetic field experiences a torque but no net force. an iron nail near a bar magnet, however, experience a force of attraction in addition to a torque. why?

Answer 1

In a uniform magnetic field, the two poles of the magnetic needle experience equal and opposite forces. In other words, the force at one end nullifies the force at the other end. Hence, the needle experience only the torque due to the magnetic field and there is no force. But, when an iron needle is placed close to a bar magnet, magnetization takes place. Moreover, since the poles do not experience a uniform magnetic field, the two poles of the magnet experience forces of unequal magnitude although the direction is different for each pole. So, apart from torque, the magnetized needle experiences a force.

Answer 2

A magnetised needle in a uniform magnetic field experiences a torque but no net force because the forces experienced by it are equal in magnitude, opposite in direction and have different lines of action. Therefore, the net force is zero. However, torque is not zero.

Due to the bar-magnet the iron nail experiences a non-uniform magnetic field. Therefore, the induced magnetic moment in the nail experiences both force and torque. The induced south pole in the nail is closer to the north pole of the magnet than the induced north pole. So the net force is attractive.